1. Field of the Invention
The present invention generally relates to an AC generator control apparatus for a motor vehicle for controlling an output voltage level of an AC generator which is driven by an internal combustion engine (hereinafter also referred to as the engine for short) and which is used for electrically charging an onboard battery of the motor vehicle. More particularly, the invention is concerned with an AC generator control apparatus of the type mentioned above which can ensure improvement of battery charging efficiency while allowing the running performance as well as fuel cost performance, of the motor vehicle to be improved by controlling the output voltage of the AC generator with high reliability.
2. Description of Related Art
In conjunction with the AC generator mounted on a motor vehicle for electrically charging an onboard battery thereof, the output voltage of the generator is set to a relatively low level (e.g. at 12.5 V) when the motor vehicle is running in a steady state at a relatively high speed. This is done with a view to improving or enhancing the running performance as well as the fuel cost performance of the motor vehicle by reducing a load imposed on the engine by the generator.
On the other hand, when the engine (and hence, the motor vehicle) is decelerated, the output voltage of the generator is changed over to a relatively high level (normal voltage level of, e.g., 14.5 V) in order to ensure sufficient braking efforts.
As an example of an AC generator control apparatus for controlling the electric power generated by the AC generator depending on the operation state of the engine of the motor vehicle in the manner mentioned above, there is known an apparatus which is disclosed, for example, in Japanese Utility Model Publication No. 28190/1992 (Japanese Utility Model Laid-Open No. 61-141945. For more particulars of this known control apparatus, reference should be made to the above-mentioned publication.
However, the AC generator control apparatuses known heretofore suffer shortcomings, which will be described below. The conventional control apparatus is so designed as to change, simply, the output power of the AC generator depending on the engine operation state of the motor vehicle. As a consequence, if the electric load of the motor vehicle increases with the engine in the steady running state (which will naturally be accompanied with a lowering of the battery voltage below a predetermined value or level), the generator starts to generate electric power to charge the battery. Thus, there may undesirably arise a situation which the engine load is not mitigated, even though in the steady state operation, presenting an obstacle to realization of desired running performance of the motor vehicle.
To cope with the problem mentioned above, there has already been proposed an AC generator control apparatus which is so designed as to change over the output voltage of the generator to a relatively high level to promote the charging of the battery when the motor vehicle is running at a speed higher than a predetermined value. Also, when the motor vehicle is running at a speed lower than the predetermined value, the output voltage of the generator is changed over to a lower level, to thereby suppress to a minimum the frequency at which the battery is charged. In that case, when it is detected that the speed of the motor vehicle exceeds a predetermined value, the level of a signal for controlling the output voltage of the generator is changed over from a low (L) level to a high (H) level. In other words, the control signal for controlling the output voltage of the generator has only two discrete levels (i.e., "L" level and "H" level).
Additionally, as is disclosed in Japanese Utility Model Publication No. 28190/1992, there has also been proposed a control apparatus for changing the electric power generated by the generator in dependence on an output signal produced by a shift position sensor which is adapted to detect shift positions indicating gear ratios of the transmission of the motor vehicle. More specifically, with this known control apparatus for the AC generator, such control is performed that when the motor vehicle is running in a high-speed steady state in which the output voltage of the generator is set at a relatively low level, a reference vehicle speed serving as a threshold value in a decision process for increasing the output voltage (or output power) of the generator in dependence on the speed of the motor vehicle is increased as the gear ratio indicated by the detected shift position signal is higher. In this case, the reference speed of the motor vehicle is increased by changing the predetermined value used for deciding the vehicle speed in dependence on the effective gear ratio as detected.
As can be seen from the above description of the AC generator control apparatuses known heretofore, the output voltage of the generator is controlled or changed on the basis of only the vehicle speed or alternatively the engine rotation number (rpm) which is one of the parameters indicating the operation state of the engine or the motor vehicle.
Further, in conjunction with the generator control apparatus disclosed in the utility model publication mentioned previously in which the output voltage of the generator is changed over to higher level depending on the gear-ratio shift positions, it has to be pointed out that the signal indicative of the engine rotation number is used only for the purpose of inhibiting the output power of the generator from being changed when the transmission of the motor vehicle is in the neutral position. In other words, the engine rotation number, which plays a very important role in controlling the output voltage of the generator, is not straightforwardly utilized for changing the output voltage of the generator. Such being the circumstances, with the AC generator control apparatus disclosed in the aforementioned publication, it is difficult or impossible to detect the operation state of the engine of the motor vehicle with high accuracy and reliability, which ultimately presents a primary cause for degradation of reliability and fidelity of the output power control of the generator.
As will now be understood from the foregoing discussion, the AC generator control apparatuses for the motor vehicle proposed heretofore suffer a drawback in that the reliability of the AC generator control remains low because the control resorts to only the gear-ratio shift position and/or the motor vehicle speed. Another problem of the hitherto known AC generator control apparatuses is that when the state in which the output voltage of the generator is set relatively low continues to exist, degradation will be involved in the battery charging efficiency.